Recently, highly efficient transmission by digitalization has been often employed in wireless communication. When a multi-phase modulation scheme is applied to the wireless communication, a technique is important according to which power leak from adjacent channels is reduced by suppressing a non-linear distortion by linearizing the amplification property especially of a transmission power amplifier on the transmitting side.
When the improvement of the power efficiency is promoted by using an amplifier having low linearity, a technique of compensating the non-linear distortion caused by the low linearity is indispensable.
FIG. 1 is a block diagram of an example of a transmitting apparatus in a conventional wireless communication apparatus. A transmission signal generating apparatus 1 sends a serial digital data train. A serial/parallel converter (S/P converter) 2 sorts the digital data train alternately one bit by one bit and converts the sorted bits into two types signals that are an in-phase component signal (I signal: In-Phase component) and a quadrature component signal (Q signal: Quadrature component).
A D/A converter 3 converts each of the I signal and the Q signal into analog base band signals and inputs the converted signals into a quadrature modulator 4. The quadrature modulator 4 multiplies the inputted I signal and the inputted Q signal (transmission base-ban$ signals) respectively by a reference carrier wave 8 and a carrier wave obtained by shifting the phase of the reference carrier wave 8 by 90°, executes quadrature conversion by adding the multiplication result, and outputs the conversion result.
A frequency converter 5 mixes a quadrature modulation signal and a local oscillation signal and converts the frequency thereof into a radio frequency. A transmission power amplifier 6 amplifies the power of a radio frequency signal outputted from the frequency converter 5 and radiates the amplified signal into the air from a aerial wire (antenna) 7.
In this case, in mobile communication such as W-CDMA, the transmission power of a transmitting apparatus is large such as 10 mW to several 10 mW and input/output property (having a distortion function f(p)) of the transmission power amplifier 5 is non-linear as indicated by a dotted line of FIG. 2. Because of this non-linear property, a non-linear distortion is generated and a frequency spectrum around a transmission frequency of has a side lobe indicated by a solid line b that is raised from the property “a” of the dotted line of FIG. 3. Therefore, leaking is generated into adjacent channels and adjacent interference is generated. That is, due to the non-linear distortion shown in FIG. 2, the power that is the transmission wave leaking to an adjacent frequency channel becomes large as shown in FIG. 3.
ACPR (Adjacent Channel Power Ratio) that represents the magnitude of leaking power is a ratio of the power of the channel of interest that is the area of a spectrum between lines A and A′ of FIG. 3 and adjacent leaking power that is the area of a spectrum that leaks to an adjacent channel between lines B and B′. Such leaking power is a noise to other channels, degrades the communication quality of those channels, and, therefore, is strictly regulated.
The leaking power is, for example, small in a linear region (see a linear region I of FIG. 2) and large in a non-linear region II of a power amplifier. To make an amplifier a high-power transmission power amplifier, the linear region I needs to be wide. However, for this, an amplifier that has a capacity exceeding a capacity actually needed is necessary. A problem has arisen that such an amplifier is disadvantageous in terms of cost and the size of the apparatus. Therefore, a distortion compensating function that compensates the distortion of the transmission power is provided for a wireless apparatus.
FIG. 4 is a block diagram of a transmitting apparatus including a digital non-linear distortion compensating function. A digital data cluster (transmission signal) sent from the transmission signal generating apparatus 1 is converted into the two types of I signal and Q signal in the S/P converter 2 and, as a preferred example, the signals are inputted into a distortion compensating unit 9 constituted of a DSP (Digital Signal Processor).
As shown being enlarged in the lower portion of FIG. 4, the distortion compensating unit 9 includes a distortion compensation coefficient storing unit 90 that stores a distortion compensation coefficient h(pi) corresponding to the power pi (i=0 to 1023) of a transmission signal x(t), a pre-distorting unit 91 that applies a distortion compensating process (pre-distorting) to the transmission signal using the distortion compensation coefficient h(pi) corresponding to the power level of the transmission signal, and a distortion compensation coefficient calculating unit 92 that compares the transmission signal x(t) with a demodulated signal (feedback signal) y(t) demodulated by a quadrature wave demodulator described later, that calculates the distortion compensation coefficient h(pi) such that the difference obtained by the comparison is zero, and that updates the distortion compensation coefficient of the distortion compensation coefficient storing unit 90.
The signal applied with the distorting process by the distortion compensating unit 9 is inputted into the D/A converter 3. The D/A converter 3 converts the inputted I signal and the inputted Q signal into analog base band signals and inputs the base band signals into the quadrature modulator 4. The quadrature modulator 4 multiplies the inputted I signal and the inputted Q signal respectively by the reference carrier wave 8 and a carrier wave obtained by shifting the phase of the reference carrier wave 8 by 90°, executes quadrature modulation by adding the multiplication results, and outputs the modulation result.
The frequency converter 5 mixes a quadrature modulation signal and a local oscillation signal and converts the frequency thereof. The transmission power amplifier 6 amplifies the power of a radio frequency signal outputted from the frequency converter 5 and radiates the amplified signal into the air from a aerial wire (antenna) 7.
A portion of the transmission signal is inputted into a frequency converter 11 through a directional coupler 10. The frequency converter 11 converts the frequency of the portion and the portion is inputted into a quadrature wave demodulator 12. The quadrature wave demodulator 12 multiplies the transmission signal respectively by the reference carrier wave and a signal obtained by shifting the phase of the reference carrier wave by 90°, thereby, executes quadrature modulation, restores the I signal and the Q signal of the base band on the transmission side, and input these signals into an A/D converter 13.
The A/D converter 13 converts the inputted I and Q signals into digital signals and inputs the digital signals into the distortion compensating unit 9. The distortion compensation coefficient calculating unit 92 of the distortion compensating unit 9: compares the transmission signal before the distortion compensation with the feedback signal demodulated by the quadrature wave demodulator 12, by an adaptive signal process using an LMS (Least Mean Square) algorism; calculates the distortion compensation coefficient h(pl) such that the difference obtained in the comparison is zero; and, thereby, updates the coefficient stored in the distortion compensation coefficient storing unit 90. Thereafter, by repeating the above operations, the non-linear distortion of the transmission power amplifier 6 is suppressed and, thereby, the leaking power to the adjacent channels is reduced.
An exemplary configuration used when distortion compensating process by the adaptive LMS as shown in FIG. 5 is executed is described in, for example, Patent Document 1 as an example configuration of the distortion compensating unit 9 in FIG. 4.
In FIG. 5, the pre-distorting unit 91 of FIG. 4 corresponds to a multiplier 15a and multiplies the transmission signal x(t) by a distortion compensation coefficient hn-1(p). The transmission power amplifier 6 is corresponded by a distortion device 15b including the distortion function f(p).
A portion including the frequency converter 11, the quadrature wave demodulator 12, and the A/D converter 13, that feeds back an output signal from a transmission power amplifier 15b in FIG. 4 is shown as a feedback system 15c in FIG. 5.
In FIG. 5, the distortion compensation coefficient storing unit 90 in FIG. 4 is constituted of a look up table (LUT) 15e. The distortion compensation coefficient calculating unit 92 of FIG. 4 that produces an update value for the distortion compensation coefficient stored in the look up table 15e is constituted of a distortion compensation coefficient calculating unit 16.
In the distortion compensating apparatus having the configuration shown in FIG. 5, the look up table 15e stores a distortion compensation coefficient for canceling the distortion of the transmission power amplifier 6 that is the distortion device 15b corresponding to each piece of power, that is discrete, of the transmission signal x(t).
When inputted with the transmission signal x(t), an address generating circuit 15d calculates the power p(=x2(t)) of the transmission signal x(t), produces an address that uniquely corresponds to the calculated power p(=x2(t)) of the transmission signal x(t), and outputs the address as designating information of a reading address.
A distortion compensation coefficient hn-1(p) stored in this reading address is read from the look up table 15e and is used for the distortion compensating process in 15a. 
The update value for updating of the distortion compensation coefficient stored in the look up table 15e is calculated by the distortion compensation coefficient calculating unit 16.
That is, the distortion compensation coefficient calculating unit 16 is configured including a conjugate complex signal output unit 15f and multipliers 15h to 15j. A subtracter 15g outputs the difference e(t) between the transmission signal x(t) and the feedback demodulated signal y(t). The multiplier 15h multiplies the distortion compensation coefficient hn-1(p) and y*(t) and obtains an output u*(t) (hn-1(p)y*(t)). The multiplier 15i multiplies the difference output e (t) of the subtracter 15g and u*(t). The multiplier 15j multiplies a step size parameter μ and the output of the multiplier 15i. 
An adder 15k adds the distortion compensation coefficient hn-1(p) and the output μe(t)u*(t) of the multiplier 15j and obtains the update value of the look up table 15e. 
This update value is stored at a writing address (AW) that the address generating circuit 15d designates as the address that uniquely corresponds to the power p(=x2(t)) of the transmission signal.
Though the reading address and the writing address are the same address, the reading address is delayed by a delaying unit 15m and is used as a writing address because the time for the calculation, etc., is necessary until the update value is obtained.
Delaying units 15m, 15n, and 15p add to the transmission signal x(t) a delay time D that is from the time when the transmission signal is inputted to the time when the feedback demodulated signal y(t) is inputted into the subtracter 15g. The delay time D set in the delaying units 15m, 15n, and 15p is determined such that, for example, D=D0+D1 is satisfied where a delay time in the transmission power amplifier 15b is D0 and a delay time of the feedback system 15c is D1.
The above configuration executes the calculation expressed below.hn(p)=hn-1(p)+μe(t)u*(t)e(t)=x(t)−y(t)y(t)=hn-1(p)x(t)f(p)u*(t)=x(t)f(p)=hn-1(p)y*(t)p=|x(t)|2 where x, y, f, h, u, and e are complex numbers and “*” indicates a conjugate complex number.
By executing the above calculation process, the distortion compensation coefficient h(p) is updated such that a difference signal e(t) between the transmission signal x(t) and the feedback demodulated signal y(t) becomes the minimum, the coefficient h(p) is finally converged to the optimal distortion compensation coefficient value, and the distortion of the transmission power amplifier 6 is compensated. Patent Document: PCT International Publication No. WO2003/103163